Skeptics of the idea that the coronavirus is spreading through the air – including many World Health Organization experts – have cited one missing piece of evidence: evidence that floating respiratory droplets called aerosols live virus, and not just fragments of genetic material.
Now a team of virologists and aerosol scientists has produced exactly that: confirmation of infectious virus in the air.
“This is where people have converted,” said Linsey Marr, an expert in airborne viruses who were not involved in the work. “There is clear evidence that there is an infected virus in aerosols.”
A research team at the University of Florida managed to isolate the live virus from aerosols collected at a distance of seven to 16 feet from patients hospitalized with Covid-19 – beyond the six feet recommended in social distance guidelines.
The findings, which were posted online last week, have not yet been reviewed by peer review, but have already caused some uproar among scientists. “If this is not a smoke pistol, then I do not know what is,” Drs. Marr tweeted last week.
But some experts said it was not yet clear how much of the virus had returned enough to cause infection.
The investigation was precise. Aerosols are by definition minute, and measure only a maximum of five micrometers through; evaporation can make them even smaller. Attempts to catch these delicate drops usually damage the virus they contain.
“It’s very difficult to sample biological material from the air and have it viable,” said Shelly Miller, an environmental engineer at the University of Colorado Boulder who researches air quality and diseases in the air.
“We need to be sensible about sampling biological material so that it’s more like how you can inhale it.”
Earlier attempts were voiced at one stage or another in the process. For example, one team attempted to use a rotating drum to suspend aerosols, and showed that the virus remained infected for up to three hours. But critics argued that these circumstances were experimental and unrealistic.
Other scientists used gelatin filters such as plastic and glass tubes to collect aerosols over time. But the force of the air shrank the aerosols and shattered the virus. Another group managed to isolate the live virus but did not show that the isolated virus could infect cells.
In the new study, researchers invented a sampler that uses pure water vapor to enlarge the aerosols enough that they can be easily collected from the air. Instead of letting these aerosols transfer, the device immediately transports them in a liquid rich in salt, sugar and protein, which retains the pathogen.
“I’m impressed,” said Robyn Schofield, an atmospheric chemist at the University of Melbourne in Australia who measures aerosols across the ocean. “It’s a very sensible measurement technique.”
As editor of the journal Atmospheric Measurement Techniques, Dr Schofield is familiar with the available options, but she said she had not seen anyone who could match the new one.
The researchers had previously used this method to test air from hospital rooms. But in those attempts, other drifting respiratory viruses grew faster, making it difficult to isolate the coronavirus.
This time, the team collected air samples from a room in a ward dedicated to Covid-19 patients at the University of Florida Health Shands Hospital. None of the patients in the room were subjected to medical procedures known to generate aerosols, of which the WHO and others have disputed the primary source of airborne virus in a hospital setting.
The team used two samplers, one about seven feet from the patients and the other about 16 feet from them. The scientists were able to collect viruses at both distances and then show that the virus they had plucked from the air could infect cells in a laboratory.
The genomic sequence of the isolated virus was identical to that of a swab of a newly admitted symptomatic patient in the room.
The room had six air changes per hour and was equipped with efficient filters, ultraviolet radiation and other safety measures to inactivate the virus before the air was re-introduced into the room.
That may explain why the researchers found only 74 virus particles per gallon of air, said John Lednicky, the team’s lead virologist at the University of Florida. Indoor spaces without good ventilation – such as schools – could accumulate much more airborne virus, he said.
But other experts said it was difficult to extrapolate the findings to estimate an individual’s risk of infection.
“I’m just not sure these numbers are high enough to cause an infection in anyone,” said Angela Rasmussen, a virologist at Columbia University in New York.
“The only conclusion I can draw from this paper is that you can culture viable virus from the air,” she said. “But that’s no small thing.”
Several experts note that the distance at which the team found virus is much farther than the six feet recommended for physical distance.
“We know that inside, those distance rules no longer matter,” said Drs. Schofield. It takes about five minutes for small aerosols to pass through the room, even in still air, they added.
The minimum of six feet is “misleading because people think they are protected indoors and they really are not,” she said.
That recommendation was based on the idea that “large ballistic-type ballistic droplets” were the only remnants of the virus, said Drs. Marr. The more distance people can maintain, the better, they added.
The findings should also push people to take precautionary measures for airborne transmission such as improved ventilation, said Seema Lakdawala, an expert on respiratory virus at the University of Pittsburgh.
“We all know that this virus can transmit through all of these ways, but we are only focusing on a small subset,” said Drs. Lakdawala.
She and other experts noted one strange aspect of the new study. The team reported finding just as much viral RNA as they did infectious virus, but other methods generally found about 100 times more genetic material.
“When you do nasal swabs or clinical samples, there is a lot more RNA than infected virus,” said Dr Lakdawala.
Dr. Lednicky has received emails and phone calls from researchers worldwide asking about that finding. He said he would check his numbers again.
But ultimately, he added, the exact figures may not matter. “We can grow the virus out of thin air – I think it should be the most important lessons for home,” he said.
